Electrohydrodynamic jet printed photonic devices

ABSTRACT

A method of fabricating a thin film structure includes printing, using an electrohydrodynamic jet (e-jet) printing apparatus, a first layer comprising a first liquid ink, such that the first layer is supported by a substrate, curing the first layer; printing, using the e-jet printing apparatus, a second layer comprising a second liquid ink, such that the second layer is supported by the first layer, and curing the second layer.

CROSS-REFERENCES TO RELATED APPLICATIONS

This application is a divisional application of U.S. non-provisionalapplication entitled “Electrohydrodynamic Jet Printed Photonic Devices,”filed Aug. 6, 2021, and assigned Ser. No. 17/396,043, which claimed thebenefit of U.S. provisional application entitled “ElectrohydrodynamicJet Printed Photonic Devices,” filed Aug. 6, 2020, and assigned Ser. No.63/062,389, the entire disclosures of which are hereby expresslyincorporated by reference.

STATEMENT REGARDING FEDERALLY SPONSORED RESEARCH OR DEVELOPMENT

This invention was made with government support under Contract No.1727894 awarded by the National Science Foundation (NSF). The governmenthas certain rights in the invention.

BACKGROUND OF THE DISCLOSURE Field of the Disclosure

The disclosure relates generally to photonic devices.

Brief Description of Related Technology

One-dimensional photonic crystals (1DPCs) consist of alternating layersof high and low refractive index materials with an optical thickness onthe order of the wavelength of the incident light. As optical filters ormirrors, the transmittance or reflectance of light by 1DPCs can be tunedby adjusting the sequence, thickness, and refractive index in the stack.1DPCs have found wide ranging applications; from conventional lasers andoptical filtering to novel mechanical and chemical sensing devices.1DPCs have been fabricated by physical and chemical vapor deposition,solution processes such as spin or dip coating and thermal drawing.Polymeric 1DPCs have attracted attention recently due to their potentialfor simplified processing, as well as freedom to design chemically- andstructurally-derived capabilities for new sensory applications.

Creating arrays of 1DPC elements (pixel filters) typically involves manycostly lithographic steps. For instance, arrays of 1DPCs for imagingapplications with pixel sizes of 30×30 μm were created withphotolithographic masking processes, achieving a 2×2 array with each ofthe 4 pixels having a different optical response. Expanding to a largermultispectral or hyperspectral array with each element having adifferent response involves a corresponding increase in maskingprocesses (e.g., 9 for a 3×3 array, 16 for a 4×4, etc.). Because adifferent optical response also involves a different thickness for eachlayer within each pixel, the number of deposition steps scales at thesame rate.

Emerging additive manufacturing (AM) processes have been applied to thecreation of photonic crystals with single and multiple materials atvarious length scales. At the mesoscale, fused deposition printing and aphotonic crystal block copolymer were combined to produce 3D objectswith structural color. Multiple photopolymers have been used withdigital light projection (DLP) to create a single structure at themesoscale. At smaller length scales of patterning, two-photonphotopolymerization was used to realize air/polymer photonic crystals atthe sub-μm length scale that achieved response in the visible regimeafter a post-print thermal shrinking procedure. While patterned arraysof photonic crystals have been demonstrated using inkjet printing, theneed for solvent orthogonality and low viscosity inks have severelylimited the structures obtained thus far.

SUMMARY OF THE DISCLOSURE

In accordance with one aspect of the disclosure, a method of fabricatinga thin film structure includes printing, using an electrohydrodynamicjet (e-jet) printing apparatus, a first layer including a first liquidink, such that the first layer is supported by a substrate, curing thefirst layer, printing, using the e-jet printing apparatus, a secondlayer including a second liquid ink, such that the second layer issupported by the first layer, and curing the second layer.

In accordance with another aspect of the disclosure, a device includes asubstrate and a patterned stack supported by the substrate. Thepatterned stack includes a first layer supported by the substrate, thefirst layer including a first photopolymer and a second layer supportedby the first layer, the second layer including a second photopolymer.The first and second photopolymers have different refractive indices.

In connection with any one of the aforementioned aspects, the devicesand/or methods described herein may alternatively or additionallyinclude or involve any combination of one or more of the followingaspects or features. The method further includes filtering the firstliquid ink in preparation for printing the first layer. The e-jetprinting apparatus is operated in a drop-on-demand mode. The methodfurther includes printing one or more pairs of layers including thefirst and second liquid inks such that the one or more pairs of layersare supported by the second layer. The method further includes forming aphotodetector on the substrate such that the first layer is supported bythe photodetector. Printing the first layer includes controlling a pulsewidth of the e-jet printing apparatus. Printing the first layer includescontrolling a spacing of droplets deposited by the e-jet printingapparatus. Curing the first layer and curing the second layer areimplemented while the substrate remains on the e-jet printing apparatus.Curing the first layer and curing the second layer include applyingultraviolet light. Curing the first layer is implemented in a nitrogenflow. The method further includes measuring reflectance of the firstlayer, using a microspectrometer integrated with the e-jet printingapparatus, before printing the second layer. The first and second liquidinks comprise first and second photopolymers, respectively. Thepatterned stack further includes further alternating patterned layers ofthe first and second photopolymers, the alternating patterned layersbeing supported by the second patterned layer. Each of the first andsecond patterned layers has a thickness less than about 200 nm. Thepatterned stack is configured as a one-dimensional photonic crystal. Thepatterned stack is configured as a Bragg reflector. The patterned stackis one of a pixelated array of patterned stacks supported by thesubstrate. The photonic device further includes a photodetectorsupported by the substrate. The photodetector is disposed between thepatterned stack and the substrate. The substrate includes silicon. Thefirst photopolymer has a higher refractive index than the secondphotopolymer.

BRIEF DESCRIPTION OF THE DRAWING FIGURES

For a more complete understanding of the disclosure, reference should bemade to the following detailed description and accompanying drawingfigures, in which like reference numerals identify like elements in thefigures.

FIG. 1 depicts perspective, schematic views of photonic structures anddevices, such as Bragg reflectors, in accordance with several examples,as well as graphical plots of reflectance and peak reflectance for anumber of differing refractive index material combinations.

FIG. 2 depicts graphical plots of transmission and reflectance for anumber of photopolymer materials for use in the disclosed photonicstructures and devices.

FIG. 3 depicts a photographic, plan view of a color filter array printedin accordance with one example, along with graphical plots ofreflectance in comparison with spin-coated color filters.

FIG. 4 depicts a photographic, plan view of a color filter array printedin accordance with one example, along with graphical plots ofreflectance measured at various positions within each structure of thecolor filter array.

FIG. 5 depicts multiple graphical plots of profiles of layers printed inaccordance with several examples.

FIG. 6 depicts multiple graphical plots of profiles of layers printed inaccordance with several examples, along with graphical plots ofreflectance in comparison with structures formed via spin-coating.

FIG. 7 is a schematic, side view of a photonic device having a number ofstacks of photopolymer layers integrated with respective detectorelements in accordance with one example.

FIG. 8 depicts a hyperspectral camera having an array of photoniccrystal structures supported by a curved substrate in accordance withone example.

FIG. 9 depicts a photonic structure having a multi-material stack inaccordance with one example, along with a schematic view of an e-jetprinting apparatus for forming the stack and graphical plots ofreflectance for the layers of the stack.

FIG. 10 depicts graphical plots of external quantum efficiency of aphotonic device having a polymeric distributed Bragg reflector (DBR)integrated with a photodetector in accordance with one example.

FIG. 11 depicts graphical plots of the average thickness resulting fromimplementation of the disclosed methods with varying pulse widths andpitches.

FIG. 12 depicts a graphical plot of external quantum efficiency of aphotonic device in accordance with one example.

FIG. 13 depicts a schematic view of an e-jet printing apparatus andexamples of a photonic structure and device fabricated thereby, alongwith a graphical plot of reflectance therefor.

FIG. 14 depicts graphical plots relating to a contact angle parameter ofthe disclosed methods.

FIG. 15 depicts a photographic, plan view of a photonic device having anarray of multi-layer photonic structures in accordance with one example,along with graphical plots of the thickness of the layers of thestructures.

FIG. 16 depicts a photographic, plan view of an array of multi-layerphotonic structures in accordance with one example, along with graphicalplots of the thickness of the layers of the structures.

FIG. 17 is a flow diagram of a method of fabricating a photonicstructure and/or device using electrohydrodynamic jet printing inaccordance with one example.

FIG. 18 is a schematic diagram of an e-jet printing system havingrespective, inline (or otherwise integrated) stations for e-jetprinting, curing, and microspectroscopy in accordance with one example.

The embodiments of the disclosed systems, devices and methods may assumevarious forms. Specific embodiments are illustrated in the drawing andhereafter described with the understanding that the disclosure isintended to be illustrative. The disclosure is not intended to limit theinvention to the specific embodiments described and illustrated herein.

DETAILED DESCRIPTION OF THE DISCLOSURE

One dimensional photonic crystal structures and photonic devicesincluding such structures are described. Methods for fabricating suchdevices using electrohydrodynamic jet printing are also described. Thedisclosed methods use electrohydrodynamic jet (e-jet) printing toprovide a mask-free, direct-deposition method that can achieve pixelatedarrays of 1DPCs and other photonic devices. E-jet printing allows a widevariety of photopolymerizable inks and other polymer materials to beused. The photopolymerizable inks can be used in an additivemanufacturing process to fabricate 1DPC and other photonic devicesdespite having a high viscosity (e.g., greater than 500 centipose, orcP).

E-jet printing is a high-resolution AM technique that operates byapplying an electric field between a conductive nozzle and a groundedsubstrate to generate extremely high shear forces on the fluid withoutadditional wall friction. The electric field, resulting shear forces,and lack of friction create micro-jets and enable droplets down tofemtoliter volumes, resulting in spatial resolution in the sub-μm range,e.g., down to 100 nm.

Examples of e-jet process parameters are described for the creation ofthe multi-material, multi-layered structures. The optical properties ofthe materials are also quantified and the photonic response of theprinted 1DPCs are determined and tied back to capabilities of thedisclosed methods.

The configuration of the disclosed photonic devices may vary. In somecases, the one dimensional photonic crystal structures are incorporatedinto an imaging device. For instance, the disclosed photonic structuresmay be configured as a filter array of the imaging device, such as aBayer RGB array. In some cases, the imaging device may be or include ahyperspectral imaging device in which the disclosed photonic structuresare disposed adjacent photodetector sites configured to capture lightacross a broad or other desired spectrum, including, e.g., light outsideof the visible spectrum. For instance, the photonic structures may bemonolithically integrated on top of the photodetector sites.

The disclosed devices may be configured as filters in hyperspectralimaging and other detectors. As described herein, the disclosed devicesmay include periodic layers of alternating refractive index material.These layered stacks may be patterned in the plane of the detectoraccording to a desired pixel layout. But instead of using typicalmicrofabrication processes, requiring many masking and lithographysteps, to create the layered stacks, the disclosed methods use e-jetprinting to avoid the complexities and expenses of the lithographicprocess. The e-jet printing of the disclosed methods also allowsnon-conventional filter array geometries to be realized.

The disclosed methods use electrohydrodynamic jet printing to printmicroscale arrays of polymer material at the nanoscale thicknessresolution used for optical interference, allowing for the realizationof desired reflection and transmission properties. In some cases, ahigh-resolution jetting process is employed to generate localized,multi-layered structures, enabling the generation of arbitrary arrays ofinterference-based optical filters.

This disclosed devices may also include the combination of printedpixels, each pixel including a printed photonic crystal (e.g., adistributed Bragg reflector (DBR)-based mirror), and a photodetectingelement. In some cases, the photodetecting elements, which may includeorganic and/or inorganic sensing layers, are monolithically integratedwith the printed mirrors and filters. Because the light detectingelements may also be printed (e.g., by organic vapor jet printing, inkjet printing, contact transfer printing, etc.), the disclosed methodsenable the realization of an all-printed hyperspectral imaging system.The organic nature of both the printed filter and photodetectingelements also enables deposition onto curved and flexible substrates.This in turn enables the realization of imaging systems with reducedimage acuity loss at image edges and reduction in volume and weight ofthe system through simplification of the optical path (e.g., removal ofimage flattening optics).

Although described in connection with one dimensional photonic crystalstructures and imaging and filtering devices including such structures,the disclosed methods and devices may be used to fabricate a widevariety of photonic and other devices, such as electronic devices. Forexample, the disclosed methods and devices may be used to realizecomplex patterns of optical (e.g. infrared) interference, and may thusbe integrated into infrared beacons and other devices for generatingpatterns of infrared beacons on various surfaces. Still other types ofphotonic devices may be fabricated, including, for instance, diffractiongratings, multiple layer arrays, gradient refractive index structures,plasmonic reflectors and/or color filters, doped and undoped resonantcavities, scattering layers, absorptive filters, and others. Thedisclosed methods may be used to fabricate non-photonic devices, such asmulti-layer, multi-material thin film transistors. Although described inconnection with structures having stacks of alternating layers of twomaterials, the disclosed methods and devices are not limited to bilayerstructures or alternating layer arrangements. Any number of materialsmay be deposited in the stacks or other structures.

FIG. 1 depicts a printed 1DPC device 100 in accordance with one example.The device 100 includes a number of 1DPC structures. Part A) of FIG. 1is a schematic diagram of a 1DPC structure 102 of the device 100. Thestructure 102 includes alternating layers of high (n_(H)) and low(n_(L)) refractive index polymers with the physical thickness of eachlayer (t_(H), t_(L)) determined by the center wavelength (λ₀) as well asthe refractive index of the layer. Part B) depicts the manner in whichthe photonic device 100 includes an array of various layer pair 1DPCstructures. The structures may be created via the e-jet printingprocess, examples of which are described herein. Part C) depicts thereflection comparison of a number of material combinations with the samenumber of alternating index layers (N=15) centered at λ₀=550 nmdemonstrating the increase in stop band width as well as reflectancewith increasing refractive index contrast. Simulations are conductedusing a high refractive index (n_(s)=3.98) silicon substrate for moredirect comparison to the printing process. Part D) depicts the peakreflectance achieved at the center wavelength (λ₀=550 nm) for an ideal1DPC structure for an increasing number of layers. The shaded regionsimulates one, two, and three-layer stacks, but other arrangements maybe included or deposited. For instance, in other cases, the structure ordevice may include one or more layers composed of, or otherwiseincluding, a material, such as another photopolymer material, other thanthe two materials that form the alternating index layers.

In the filter array of FIG. 1 , the refractive index contrast betweenthe materials in the stack controls the width of the stop band. Forinstance, a low contrast allows for creation of narrow bands such asthose used in notch filters, while a higher contrast may provide abroadband reflector. To minimize parasitic absorption and a resultingloss in reflection, the materials may be selected to have hightransmission across a desired wavelength range.

Several trade-offs regarding ink composition may be navigated inbalancing process and application considerations. Titania (TiO₂) andsilica (SiO₂) sol-gels have previously been used in dip-coated photoniccrystals that achieved significant refractive index contrast of Δn=0.86(n_(TiO2)=2.34 and n_(SiO2)=1.48). Silica and titania-silica inks werealso used in a direct-write AM process to produce optical quality glasscomponents with varying refractive indices. In comparison, fluorinatedpolymers have refractive indices as low as n of about 1.3, while thosewith large aromatic rings or sulfur groups can have indices as high as nof about 1.7, yielding a contrast maximum of Δn=0.4. Refractive indexcontrast in polymeric 1DPCs ranged from Δn=0.07 for apolystyrene/polyvinylpyrrolidone (PS/PVP) combination to Δn=0.18 for apolyvinyl carbazole/cellulose acetate (PVK/CA) combination.

A variety of photopolymers and other liquid inks may be used in thedisclosed methods and devices. Photopolymers are a mature class ofmaterials with a wide range of commercially available compositions, withrefractive indices ranging from, for instance, 1.315 (e.g., fluorinatedacrylate polymers) to greater than 1.70 (e.g., zirconia/titaniananoparticle doped acrylates). In terms of refractive index contrast,the polymeric system examples discussed herein are significantly lowerthan the sol-gel systems, thereby involving more layers to achieve thesame optical response. However, in the context of the e-jet printingfabrication methods, the ability to print multiple layers without havingto remove the substrate is useful for both printing speed and reductionof errors due to re-registration when replacing the substrate. Althoughdescribed in connection with photopolymers, the disclosed methods anddevices are not limited to layers composed of photopolymers or otherpolymers. The layers may include additional or alternative materials,including, for instance, nanoparticle-based liquid inks and variousinorganic materials.

The use of oxide sol-gels involves a multi-step curing process,including annealing at over 400° C., while the PS/PVP and PVK/CA polymercombinations both involve low temperature thermal cures around 100° C.In contrast, photopolymers typically require a sub-30 second exposure toUV light to solidify the film, thereby allowing multiple layers to becured in-situ on the printing apparatus. The easier processability andrelatively high refractive index contrast compared to benchmarkpolymeric systems make photopolymers useful candidates for use in amulti-layer, multi-material printed photonic crystal or other photonicdevice.

The following examples of photopolymers are addressed further herein:NOA170, a high index acrylate photopolymer doped with zirconiananoparticles (average diameter 8-11 nm), Loctite 3526, a moderate indexacrylate photopolymer, and NOA1348, a fluorinated low refractive indexphotopolymer. The industry-provided cured refractive indices of thesethree materials are: 1.70, 1.51, and 1.35, respectively. Otherphotopolymer materials may be used, including, for instance, variousphotopolymers doped with other metallic or dielectric nanoparticles,stimuli responsive photopolymers, and photopolymers composed of, orotherwise including, block copolymer base units including, for instance,azobenzene, epoxide groups, oxetane groups, thiol groups for attachinggold nanoparticles, and/or separate photo-initiator additive(s), such asIrgacure™, 1-Hydroxy-cyclohexylphenylketone, and similar compounds ofdifferent types (e.g., Norrish Type 1, Norrish Type 2), with responsewavelengths tunable via composition.

The reflection spectra of a 15-layer photonic crystal are calculated foreach material pair (Part C of FIG. 1 ) by solving the Fresnel equationsusing the transfer matrix method, illustrated at a convenient centervisible wavelength, λ₀550 nm, of the 1DPC. Part D of FIG. 1 demonstratesthe increase in peak reflectance with an increasing number ofalternating index layers, showing that a higher contrast allows the useof fewer layers to achieve a given level of reflectance. The proposedphotopolymer combinations achieve comparable or better reflectanceresponse as those obtained in previously reported polymeric 1DPCsystems. For instance, the photopolymer combination of NOA170-Loctitehas very similar optical characteristics to the polyvinylcarbazole-cellulose acetate (PVK-CA) combination. The spin-coated PVK-CAsystem is thus used hereinbelow as a model system for comparison to theprinted structures of the disclosed devices.

FIG. 2 presents variable angle spectroscopic ellipsometry (VASE) andtransmission spectroscopy to measure the refractive index andtransmission across the visible to near infrared (400-800 nm) spectrumranges of the photopolymers (NOA170, NOA1348, Loctite 3526) and thethermally-cured polymers (PVK and CA). The NOA materials arecommercially provided with refractive index values of 1.70 for NOA170,1.51 for Loctite 3526, and 1.348 for NOA1348 at the sodium D line (589nm). Measured values at 589 nm were: n=1.72, 1.52, and 1.35 for NOA170,Loctite 3526, and NOA1348, respectively. The values of n=1.48 and 1.66for CA and PVK, respectively, also agree well with literature. Note thatfor films that are 100 nm thick, any large particle could lead to asignificant amount of surface roughness. Filtering was useful inconnection with one of the inks addressed herein, Loctite 3526. Forexample, a 0.22 μm filter may be used to remove large oligomerentanglements or resin particles that can otherwise cause clogging ofthe small diameter nozzle prior to printing. A potential drawback tofiltering the inks is that the refractive index or transmissionproperties may change as a result. For the Loctite, a slight drop inrefractive index of 0.01 and an increase in transmission of 2% wereobserved. Overall, the measured refractive index contrast as well ashigh transmission across the visible to NIR spectrum (>80% for allmaterials tested) make these inks useful candidates for use in a printedphotopolymer photonic crystal or other photonic device.

The disclosed methods are configured to produce a thin, uniform layer ofpolymer over a defined area. Once a polymer is deposited and cured, itthen becomes the substrate for the next layer. Whether a polymer willdeposit effectively, and ultimately merge to form a film, is determinedin part by the surface energy of both the liquid polymer and the solidsurface. In some cases, the first layer of a 1DPC or other photonicdevice is the higher refractive index material. Thus, the followingcured-uncured polymer interactions are addressed herein: interactionbetween the high index photopolymer and the printing surface (e.g.,silicon), interaction between the liquid lower index polymer on a curedhigher index layer, and interaction between the liquid higher indexpolymer on a cured lower index layer. If these interactions are known,it is possible to predict which material combinations are likely tomerge to a film using e-jet deposition. Higher order structures may thusbe realized, including, for instance, stacks of three, five, seven, etc.layers. The results and discussion of solid surface energy and liquidsurface tension at the macro and microscale are addressed below. Insummary, the relatively low solid surface energy of the low refractiveindex materials (e.g., Loctite 3526 and NOA1348) coupled with the highliquid surface tension of the high refractive index material (e.g.,NOA170) make it challenging to form a third layer (high index on lowindex). This is likely due to the low work of adhesion to the lowsurface energy polymer substrates as well as high work of cohesion ofthe NOA170 to itself, preventing the spreading of deposited ink. The lowsurface energy is likely due to the fluorinated groups used to decreasethe effective refractive index of the low index photopolymer. Thecorresponding increase of film roughness due to partial merging, and itseffect on the optical properties of the layers, is discussed furtherbelow.

A variety of different single and multi-layer photopolymer filmstructures may be fabricated via the disclosed methods. The e-jetprinting of the disclosed methods may be used to print high resolutionpatterns of multiple materials by applying a high voltage (e.g.,200-1000 V but other voltages may be used) between a small diameter,conductive nozzle and a grounded substrate. This forms what is known asa Taylor cone of material at the nozzle orifice which, when pulsed witha varying voltage, ejects a drop which is smaller than the diameter ofthe nozzle.

There are several printing modes that are possible with the e-jetapparatus, including continuous cone-jet, multi-jet, and drop-on-demand.In drop-on-demand mode, the droplets are deposited at high frequency andthen merge within milliseconds of contacting the surface to form first aline and then a film. Due to the extremely thin layers and high spatialresolution involved for creating the 1DPCs or other disclosed devices,drop-on-demand printing is useful for its droplet volume and placementprecision. Other modes, such as the continuous cone-jet mode, may beused, but may deposit excess material in one location and thus small,thin layers would be limited by stage speed.

The spacing and sizing of the drops determines the resulting filmthickness as well as surface roughness. In some cases, a 1 μm nozzle isused with drop-on-demand printing, but other nozzle sizes may be used.The droplets merge within milliseconds of contacting the surface to formfirst a line and then a film. Once an array of drops has been printed,the array is shuttled to a curing station where nitrogen gas is flowedover the surface and the surface is exposed to 365 nm UV light (e.g.,via a UV lamp). The nitrogen gas is useful for full curing of NOA170 and1348 as the photopolymerization reactions are oxygen-inhibited. In othercases, nitrogen-based curing is not used. For example, the Loctite 3526photopolymerization is not oxygen inhibited. The curing technique(s) mayvary in alternative or additional ways, including, for instance, use ofa UV laser, curing under visible radiation, curing under near infraredradiation, and curing under near infrared radiation with the use ofup-converting nanoparticles.

In some cases, the e-jet apparatus is outfitted with two nozzles, whichprovide several added benefits over a single nozzle system. Due to thebi-material composition of the 1DPCs, a dual nozzle system allows forefficient switching between material sources, reducing registrationerrors caused by switching nozzles. The dual nozzle configuration mayalso increase production throughput, which reduces evaporative cloggingof the individual nozzles.

The droplet diameter may be used to achieve a desired film thickness.For instance, to achieve an approximately 100 nm thick film, the dropletdiameter deposited may be between 1.0 and 2.5 μm, depending on thepolymer being used. However, a wide variety of other diameters may beused.

NOA170 was first deposited as a single uniform layer and processparameters were adjusted to modulate thickness and, therefore, opticalresponse as shown in Part B of FIG. 2 . Three printed examples weremeasured with both atomic force microscopy (AFM) and opticalmicroreflectance (OM), with the AFM-measured thicknesses of the threeexamples (78 nm, 105 nm, and 155 nm) obtained by modulating the spacing,or pitch, of the droplets. Further details are provided below. Thesethicknesses, along with wavelength-dependent refractive index data, werethen used as inputs for the TMM simulation. Reflectance spectracollected from 15 μm spots in the center of the AFM-scanned area matchedclosely to the simulation, suggesting that OM can be used to accuratelydetermine film thickness in situ and more conveniently and rapidly thanAFM.

Two examples of bilayer structures were fabricated and tested. Theexamples involved a combination of NOA170 and NOA1348, and a combinationof NOA170 and Loctite 3526. Uniform layers of each are obtained at lowthicknesses (e.g., less than 200 nm for both). Optical simulations,taking into consideration the index of each of the layers, were used tocompare the measured reflectance spectra and close matching for bothmaterial sets.

FIG. 3 depicts an example of an all-printed Bayer filter array 300having a bilayer configuration. In this example, bilayer structures 302of the array 300 included layers composed of, or otherwise including,Loctite 3526. Other materials or material combinations may be used inthe bilayer structures 302. While other combinations can be printed in abilayer configuration, Loctite 3526 formed more uniform films thanNOA1348, and was thus used as an example to demonstrate the ability tocreate the all-printed Bayer filter array 300 of FIG. 3 .

In the printed Bayer filter bi-layer configuration shown in FIG. 3 , anoptical micrograph (50×) of the printed Bayer array composed ofNOA170-Loctite 3526 dual layers demonstrated uniformity of pixel size(40 μm×40 μm) and color with blue, green, and red reflected lightvisible. Thicknesses of each layer along with standard deviations acrossall examples (e.g., a number of measured samples, N, being 4 to 9) foreach color pixel are denoted in the graphic of FIG. 3 and in Table 1below.

TABLE 1 Geometric and Optical Comparison between E-jet Printed (NOA170-Loctite 3526) and Spin-Coated (PVK-CA) examples in FIG. 3 Layer 1 Layer1 Layer 2 Layer 2 Peak Peak Thickness σ_(RMS) Thickness σ_(RMS)Reflectance Reflectance (nm) (nm) (nm) (nm) (%) (nm) E-jet Printed Layer1: NOA170 Layer 2: Loctite Blue (N = 4) 92.7 ± 3.1 8.0 ± 0.4 180.9 ± 6.512.8 ± 1.5 37.9 ± 0.9 430.5 ± 4.0 Green (N = 9) 132.8 ± 3.6  8.8 ± 1.3 189.2 ± 12.5 17.8 ± 3.4 33.4 ± 1.2  514.1 ± 15.0 Red (N = 5) 86.7 ± 5.19.3 ± 1.1 132.5 ± 7.3 17.7 ± 4.6 25.1 ± 1.0 688.8 ± 6.5 Spin-CoatedLayer 1: PVK Layer 2: CA Blue (N = 5) 92.0 ± 1.5 5.5 ± 1.4 183.9 ± 6.9−0.6 ± 1.4 42.5 ± 0.7 436.2 ± 7.2 Green (N = 5) 128.2 ± 2.4  −0.6 ± 2.6 183.1 ± 1.7  0.8 ± 1.6 40.2 ± 0.7  468.0 ± 11.2 Red (N = 5) 84.7 ± 1.19.7 ± 0.9 127.1 ± 1.4  0.1 ± 1.8 25.7 ± 0.1 675.6 ± 3.4

Part B of FIG. 3 depicts optical microspectroscopy results of all of Row2 and Row 3 (9 examples each) matching visual determination of reflectedcolors in Part A of FIG. 3 . Simulations (shaded region) based on highand low error (see plus/minus thickness variation in Part A of FIG. 3 )in all three dual layers shows good bounding of all measured examples.The TMM simulations incorporate surface roughness as determined by AFM.

Part C of FIG. 3 shows an optical response comparison of printedNOA170-Loctite stacks with spin-coated PVK-CA dual layers (N=5). Due tothe almost identical refractive index contrast, spin-coated stacks withsimilar thicknesses show similar optical response. Process variation(plus/minus thicknesses) and surface roughness (between 2-10 nm) for thespin-coated layers was lower and there was a resulting closer bound tothe measured data. TMM data is shown via lines in Part D of FIG. 3 .

In this example, the Bayer array 300 includes pixelated color filters,50% of which are for filtering green light, and 25% of which are for redand blue light, respectively. To match the color response of thesefilters, it is possible to design for peak reflectance at certain partsof the spectrum by varying the thickness of the high and low indexmaterials independently. The thicknesses of each layer gathered via AFMare summarized in Table 1 as well as average RMS roughness values. Theclose matching between the TMM simulation using the AFM-measuredthicknesses and the optical reflectance is consistent over a relativelylarge sample size (N=18), demonstrating the repeatability of themanufacturing technique.

FIG. 4 shows spatial variation (or uniformity) of spectral response(e.g., pixel reflectance) across a single pixel. The OM system wasutilized to take five samples, one in the center and four corners, ofblue, green, and red pixels as indicated in the optical micrographs. Inthis case, the peak reflectance of the blue and red samples had lowstandard deviations in location not exceeding a 10 nm shift. There was alarger shift in the green samples, up to 18 nm, which is attributable tothe higher degree of variation in thickness across these samples.

Part A of FIG. 4 shows an optical micrograph of a Bayer filter array 400with a zoomed region indicating the locations across the area of aspecific pixel that were measured using the OM system. Circles overlaidon the central sample show exact locations of data collection while thesquare/triangle symbols indicate the locations (Center, NW, SW, NE, SE)of corresponding information in the reflectance plots. Part B of FIG. 4shows reflectance spectra of two samples each of blue, green, and redsamples. Shaded regions indicate the standard deviation in peakreflectance location for each sample. The larger reflectance variationin the green samples can be attributed to larger thickness variations inLayer 2 (Loctite 3526) as compared to the red and blue samples.

To benchmark the printed samples, dual layers of spin-coated PVK-CA weredesigned and fabricated to have a similar optical response to theprinted structures. These results are shown in Parts C and D of FIG. 3 .Under the same illumination conditions, there is a qualitative matchbetween the microscale printed samples and macroscale spin-coatedsamples. In addition, Table 1 provides a quantitative comparison in peakreflectance between the e-jet printed and spin-coated samples. Overall,there was an 11% decrease in peak reflectance in e-jet printed samples;attributed to the higher degree of surface roughness and resultantscattering loss. There was, however, a comparable standard deviation inpeak reflectance position across all samples, driven by relatively lowvariation in thickness of 6.5 nm and 2.5 nm for printed and spin-coatedsamples, respectively.

The disclosed methods allow photonic or optical structures such as 1DPCsto be fabricated in accordance with precise specifications to achievethe desired photonic response. For example, the thickness of both thehigh and low index layers may be maintained in a repeating fashionthroughout the entire multilayered structure to generate constructiveinterference. Stochastic modelling of 1DPCs showed that the uncertaintyin thickness of both the high and low index layers remains within 10% ofthe design thickness (above or below) to maintain thickness tolerancesfor creating constructive interference at a desired wavelength. In theexamples presented in Table 1, the maximum plus and minus thicknessdeviation for any printed layer was for the second layer in the greenexamples at 12.5 nm. For a 189 nm thick film, that is a 7% deviation inthickness, thus falling within the 10% functionality threshold.

The root mean square surface roughness (σ_(RMS)) at the interfaces of a1DPC structure may also have an effect on reflectance response. Forinstance, for 1DPCs covering the visible and near infrared, thereflectance may remain constant for σ_(RMS) values under 20 nm. Thus,for the disclosed methods and devices involving visible and nearinfrared 1DPC structures, a threshold of 20 nm may be set as anacceptable limit. Returning to Table 1, the maximum measured σ_(RMS) was17.7±4.6 nm, which, taking into consideration the standard deviation, isnear the roughness limit set. This is also comparable to the PVK-CAsystem that exhibited a maximum σ_(RMS) of 9.7±0.9 nm. It should benoted that the printed examples were created without integrated processcontrol and thus thickness variation as well as surface roughness areexpected to decrease with further control.

The disclosed methods may also be used to fabricate pixelated arrays of1DPCs and/or other structures, as well as other photonic devicesincluding such pixelated arrays. In high resolution imaging, forexample, the size and sharpness (e.g., edge sharpness) of opticalfiltering elements is useful for achieving overall system capabilities.For instance, one ideal manufacturing process would be able to depositlayers composed of perfect rectangular prisms stacked on top of eachother. Consider the “blue” patterns in FIGS. 3 and 4 . The designedheight for layers 1 and 2 were 90 nm and 180 nm, respectively. Thedesigned in-plane dimensions were 40 μm by 40 μm.

A parameter such as the deposit shape factor, η, may be used to quantifythe deviation between ideal and manufactured deposits, with a value ofzero being a deposit without deviation from design specifications.

FIG. 5 depicts the cross-sectional profile of the deposits for the firstand second printed layers for the blue structure (e.g., NOA170/Loctite).The layers were measured by AFM and compared to the “ideal” structurevia the deposit shape factor, defined as the ratio of the printedcross-sectional area to the ideal cross-sectional area. The upper plotsshow the low aspect ratio of the first layer. The lower plots show thatthe deposit shape factor for the second layer is farther from 0 than forthe first layer.

In these examples, a deposit shape factor η=0.14 was found for thesecond layer, greater than the η=0.06 found for the first layer. Thedifference may be attributed to the challenge in depositing onto arelatively rough printed polymer surface. Furthermore, the tapering inthe first layer led to a smaller printing area available for the secondlayer. Thus, spatial analysis of the printed pixels was also conductedto quantify the capabilities of the manufacturing technique. Deviationsfrom designed ideal spatial values for the 40×40 μm² square pixels were0.8%, whereas 13×13 μm² pixels exhibited deviations of 4.4%. The largerspreading of smaller pixels may be attributed to challenges in balancingthickness and spatial requirements. In both cases, the results suggestthat the e-jet printing can achieve industrially relevant, patterned1DPCs with acceptable error in thickness, surface roughness, pixelsharpness, and spatial area coverage.

In view of the surface studies addressed herein, one challenge increating a layered structure for the disclosed methods and devicesinvolves overcoming the lower solid surface energy of NOA1348 andLoctite as well as the high liquid surface tension of NOA170. Inpractice, it was found that the NOA170 ink would not merge into a filmon top of NOA1348 but would partially merge into a thin film on Loctite.The higher liquid surface tension NOA170 ink may be more likely tocohere to itself versus adhere to a low solid surface energy surfacelike Loctite or NOA1348. The slightly higher solid surface energy of thefiltered Loctite over NOA1348, 19.4 mN m⁻¹ and 11.5 mN m⁻¹,respectively, may also explain the preferential merging characteristics.

FIG. 6 depicts a semi-uniform layer of NOA170 deposited on top ofLoctite, as demonstrated by the relatively high roughness of the thirdlayer (σ_(RMS)>30 nm). As evidenced by the measured spectra, theroughness can result in a significant reduction in the reflectanceresponse due to light scattering. This is further confirmed by comparingthe printed example to the response of a spin-coated sample of equallayer thickness. The surface roughness was below 10 nm for all threelayers in the PVK-CA sample and thus there was low scattering loss. Asthese results indicate, it is possible to derive both thickness androughness data from OM scans more rapidly than from AFM scans. Whilesurface roughness is the likely cause of reflection loss in thesesamples, there are other potential sources of optical loss. Due to themultilayered and multi-material nature of the 1DPCs, there is also apotential for contraction of the photopolymers, particularly during thecuring process. For example, multilayered, UV-cured acrylics have shownsignificant warping at the millimeter scale. Any variation in thicknessdue to warping could significantly affect the optical performance of the1DPCs. However, there was no noticeable warping detected in themicroscale printed multi-layers, which may be attributed to length scaledependence of the behavior.

Once cured, the photopolymers investigated here are quite durable.Developed as adhesives, the shore durometer of the NOA170 and Loctite3526 are 75 and 62, respectively. For a frame of reference, the shoredurometer of a high-density polyethylene hard hat is approximately 75.To show the durability of the optical performance of these printedphotopolymer 1DPCs, reflectance measurements were taken from severalsamples over one year after they were manufactured and only minor shiftsin the spectra were observed.

Overall, the thicknesses of the layers being deposited (between about 60to about 200 nm) are in the optimal range for near infrared (NIR) 1DPCsnear the boundary of the visible regime. A 7-layer 1DPC with similarthicknesses as in FIG. 4 (tH=100 nm and tL=145 nm) would have a peakreflectance of around 35% at 780 nm (if the example were deposited onglass). Based on the transmission results gathered, these materials willstill have good transparency at these longer wavelengths and thus thesephotonic crystals could find applicability in an array of NIR opticaldevices.

FIG. 6 depicts an example of a photonic device having a tri-layer, 1DPCconfiguration. In Part A of FIG. 6 , AFM-generated cross-sections ofprinted NOA170/Loctite 3526/NOA170 tri-layer pixel with average layerthickness, surface roughness, and top view AFM inset. In Part B of FIG.6 , zoomed cross-sections of printed layers show an ideal thickness (100nm/145 nm/100 nm) and base profile for each layer along with the depositshape factor, η. A high degree of surface roughness can be seen in thethird NOA170 layer. Part C of FIG. 6 depicts reflectance spectra and atransfer matrix simulation (TMM) of the three-layered sample ofPVK/CA/PVK (102 nm/146 nm/100 nm) showing low MSE between the TMM and OMspectra due to low surface roughness (σRMS<10 nm) in all three layers.Part D of FIG. 6 depicts the reflectance spectra and TMM ofNOA170/Loctite 3526/NOA170 with micrograph inset. Inclusion of roughnessin TMM simulations results in a closer match to the optical spectra withmean-squared error (MSE) between the optical response and simulationwith roughness being 10.2 versus 29.5 for the simulation withoutroughness. The drop in reflectance is attributed to scattering resultingfrom a high degree of surface roughness in the third printed layer.

Described herein are methods and devices that utilize photopolymericmaterials with refractive indices near the boundaries of understoodorganic maximums as the constituent materials in one dimensionalphotonic crystals and other photonic devices. Criteria for selectingappropriate material pairs, based on a negotiation of optical andinterfacial characteristics, were identified as well. Photopolymers withhigh refractive index contrast and good transmission were selected forease of processing on the printing platform. As described below,interfacial materials selection criteria may be used such that drops ofphotopolymer ink are more likely to merge to uniform layers on highsolid surface energy substrates (e.g., NOA170 and silicon), while thehigh liquid surface tension of NOA170 prevents good adhesion to lowsolid surface energy substrates such as NOA1348 and Loctite.

Electrohydrodynamic jet printing is used as a platform for depositingthe photopolymers in precise three-dimensional space with micron-scale(μm) resolution in X and Y and nanometer resolution in Z. Examples ofprinted structures were fabricated with layer thicknesses from about 60nm to about 200 nm, which allowed for modulation of light directly inthe near infrared (700-1000nm) and a significant portion of the visiblespectrum (400-700 nm). Surface roughness, due to incomplete merging of asingle layer, was high compared to other polymeric 1DPCs reportedpreviously, yet still suitable for realizing dielectric mirrors andfilters.

In some cases, the polymeric 1DPCs of the disclosed devices may beintegrated or otherwise coupled with optoelectronic structures ordevices. For example, a customized, all-printed array of 1DPCs forwavelength selection may be realized directly on a CMOS circuit,eschewing masks or solvents, thus addressing an issue with spatial andspectral resolution trade-off in the hyperspectral imaging community.Furthermore, and more in line with previous polymer-based 1DPCdevelopments, the disclosed methods and devices may allow for a largenumber of customized optical sensors to be printed onto a single chip,finding applications in fields ranging from bioanalyte to gas sensing.

Various photopolymers may be used. In the examples described herein, thephotopolymers were sourced from Norland Products (Cranbury, NJ) andHenkel Corporation (Düsseldorf, Germany). Norland Optical Adhesive 170(NOA170) has a viscosity around 5,000 cP at 25° C. while NOA1348 has aviscosity around 1,600 cP at 25° C. Loctite 3526 has a manufacturersupplied viscosity of 17,500 cP at 25° C.

Additional or alternative photopolymers may be used. For example, thelow (or lower) index layers of the disclosed devices may be composed of,or otherwise include, various fluorinated polymers. For example,fluorinated polymers, such as fluoroacrylates, fluoroepoxides,fluorooxetanes, fluorinated thiol-enes, and fluorinated vinyl ethers,may be used. The high (or higher) index layers of the disclosed devicesmay be composed of, or otherwise include, various polymers (e.g., basepolymers such as acrylates, epoxides, and oxetanes) with monomers oroligomers containing functional groups, such as sulfur (e.g., linearthioether and sulfone, cyclic thiophene, thiadiazole and thianthrene),halogens (e.g., brominated or iodinated groups may be used to increasethe refractive index), and phosphorous (e.g., phosphonates andphosphazenes) and/or high index nanoparticles, such as zirconia, titania(anatase and rutile), amorphous silicon, lead sulphide (PbS), and ZnS.The nanoparticle material(s) may be selected in accordance with thewavelengths of interest. For instance, bismuth telluride may be usefulin connection with infrared wavelengths.

Printed samples may be cured on the e-jet printing setup. In some cases,the curing was implemented using a 365 nm UV LED lamp. Alternatively oradditionally, the curing includes a nitrogen flow over the surface toprevent oxygen inhibition of the photopolymerization process.

One or more of the photopolymers may be filtered using a 0.2 μm filterprior to printing. Further details regarding the curing and other actsof the disclosed methods is provided below.

FIG. 7 depicts a photonic device 700 having an array of pixels 702supported by a substrate 704. Each pixel 702 includes a stack ofphotopolymer layers and a detector element in accordance with oneexample. The photonic device 700 may be configured as, or integratedwith, a hyperspectral or other imaging device or system.

FIG. 8 depicts an example application in which an autonomous aerialvehicle is equipped with a hyperspectral imaging system 800. In thisexample, the imaging system 800 includes a curved sensor or imagingdevice 802 for imaging over a wide field of view. The imaging device 802includes a matrix or array of pixels 804. In this case, each pixel 804has one or more organic photodetector/photonic crystal stacks. Eachstack may be configured to capture a narrow band, contiguous spectralrange for the entire image.

FIG. 9 depicts an example of a distributed Bragg reflector 900 withalternating layers of high (or higher) and low (or lower) index polymermaterials. As shown in Part A, the layers are arranged in a stack withthe high index layer adjacent a substrate supporting the layers. Part Bof FIG. 9 depicts an example of an e-jet printing apparatus used tofabricate the stack. A live image inset is also shown. Part C of FIG. 9is a graphical plot of spectral reflectance of differing thicknesses ofa single layer of high index photopolymer. The dashed lines in the plotare indicative of transfer matrix simulations. An optical micrograph ofthe sampling area is shown in an inset. Part D of FIG. 9 is a graphicalplot of reflectance of a two layer example including a high indexphotopolymer (n=1.70) and low index photopolymer (n=1.35) depositedsequentially.

FIG. 10 depicts the predicted external quantum efficiency of, in Part A,a polymeric distributed Bragg reflector (DBR) of varying layer pairsintegrated with a photodetector (e.g., a SubPc/DMQA OPD), and in Part B,higher order DBRs with varying central reflection wavelengths integratedwith the same photodetector. FIG. 10 shows the peak shifting capabilityof the disclosed structures.

The disclosed methods of electrohydrodynamic jet printing may beutilized to create multi-layer, multi-material constructs, e.g., withindividual layer thicknesses between 80-200 nm, square pixels smallerthan 40 μm across, and with surface roughness less than 20 nm, but otherthicknesses, sixes, and other parameters may be achieved. The thicknessand roughness specificity provided by the disclosed methods mayaccordingly meet the requirements of high precision optical interferencestructures while maintaining the ability to arbitrarily dictate spatialvariation in optical response via a direct additive manufacturingmethod.

FIG. 11 details how the thickness and surface roughness of the resultantthin films are controlled in accordance with one example. As describedfurther herein, the disclosed methods are capable of depositingmulti-layered structures, each layer being composed of, or otherwiseincluding, different refractive index polymers, to achieve control overthe spatial optical interference via, e.g., microscale pixels.

A number of process parameters may be used to control the averagethickness (g) and surface roughness (sq) of the layers. In FIG. 11 ,error bars indicate root-mean-square (RMS) roughness for a number ofexamples as measured by an integrated AFM. The patterns (or layers) ofthe examples were printed on bare silicon wafers using an e-jet printeroperated in drop-on-demand mode. In these cases, the drop-on-demand modeused a rastering motion. Other motions and modes may be used in othercases.

Examples of such process parameters include the pulse width (t_(p)) andpitch. In Part A of FIG. 11 , the effect of pulse width (t_(p)) onpattern thickness and roughness is plotted for a fixed pitch of 2.4 μm.In Part B of FIG. 11 , the effect of pitch on pattern thickness androughness is plotted for a fixed pulse width of 1 ms. The averagedroplet diameter D was 2.8 μm. Convex patterns resulted from pitchesmuch less than the average droplet diameter D. Uniform patterns resultedwhen the pitch was approximately equal, or similar, to the averagedroplet diameter D. Rough patterns resulted when the pitch was greaterthan the average droplet diameter D.

FIG. 12 presents the results of simulations that show that the couplednarrowband absorption of the organic photodetectors with the tunableoptical modulation of the printed 1DPCs may achieve narrow detectionpeaks. Because implementations exist in which the light detectingelements are also printed (e.g. by organic vapor jet printing, ink jetprinting, contact transfer printing, etc.), the disclosed methods andsystems enable the realization of an all-printed system with multiplexeddetection capabilities. The organic nature of both the printed filterand photodetecting elements also enables deposition onto curved andflexible substrates. This in turn enables the realization of imagingsystems with reduced image acuity loss at image edges and reduction involume and weight of the system through simplification of the opticalpath (e.g., removal of image flattening optics).

In the example of FIG. 12 , a multilayered, printed 1DPC structure isfirst printed on a glass substrate, followed by deposition of an organicphotodetector device. The photodetector device may include a bulkheterojunction (e.g., PTCBI/SubPc) active layer. The configuration ofthe stack is shown in an inset along with illumination from an externalOLED to realize a fully integrated sensing device.

The example of FIG. 12 shows that multiple detection peaks may becreated by placement of the stopband of the interference filter.Coupling of the narrowband OPD response with the tunable nature of thepolymeric 1DPC allows for an integrated sensing device, which may beuseful in a wide variety of biological or chemical applications.

The disclosed methods may be directed to fabricating vertically stacked,thin-film photonic devices using multiple materials (e.g., two or morematerials). The disclosed methods use the additive manufacturingtechniques of e-jet printing to enable material deposition on previouslyformed surfaces, by direct addition of material on existingtopographies, without requiring cleanroom facilities and the use ofmasking steps more commonly used with lithography, and less materialwaste. The e-jet printing techniques are also capable of depositing highviscosity inks (e.g., greater than 50 cP) that are useful for certainapplications, but cannot be printed using inkjet technology.Furthermore, the thermal or piezo-driven excitation used to depositmaterials in a liquid phase in inkjet printing limits the achievablespatial resolution to larger than 20 μm.

E-jet printing is a solution-based fabrication technique enablingthin-film fabrication and patterning without the planarity restrictionsof lithography and other subtractive processes. Compared to inkjettechnology, e-jet printing has a much higher spatial resolution (0.05-30μm), comparable to the resolution of lithographic processes, while alsoproviding a high degree of freedom in creating customized patterns.Complex structures can be fabricated with high controllability andprecision in desired locations from the micro- to nanoscale. E-jetprinting is also capable of depositing a wide range of fluidviscosities, e.g., from 100-105 cP up to and beyond 50000 cP, severalorders of magnitude larger than that of inkjet printing. This furtherenables flexibility in the classes of materials deposited, frombiological materials to polymers and conductive inks. Manufacturingspeed can also be increased by integrating multiple parallel printheadsdepositing multiple materials onto one platform.

FIG. 13 depicts an e-jet printer 1300 for use with the disclosed methodsin accordance with one example. The e-jet printer 1300 may include aconductive nozzle 1302 holding the build material, a conductivesubstrate 1304, and a voltage amplifier. In one example of the e-jetprinting process, a high voltage pulse is applied to the nozzle 1302 toeject a droplet of material with droplet volume related to pulse-width,tp. The e-jet printer 1300 may include multiple (e.g., two) printheads.In operation, an electric field is created by applying a voltagedifference between the nozzle tip and the grounded substrate 1304,changing the meniscus profile from a pendant shape to a cone shape,defined as a Taylor cone jet. As the field strength increases,electrostatic forces overcome ink capillary tension and the liquid buildmaterial jets from the tip of the cone to the substrate. The appliedvoltage can be pulsed with a pulse-width, t_(p), from low voltage,V_(l), to high voltages, V_(h), as shown in Part A of FIG. 13 . Customstructures may be fabricated by synchronizing the stage motion with thevoltage pulses.

The disclosed methods may use an e-jet printer in a drop-on-demandprinting mode. In that mode, the e-jet printer is capable of depositingsessile droplets at specified locations via, e.g., the above-referencedsynchronization. Alternatively or additionally, a continuousjet-printing mode may be used to deposit material on the substrate. Thecontinuous mode may be operated in a line printing manner. However, thecontinuous mode may not be as well suited for the fabrication of uniformthin films (e.g., film spatial resolution <100 μm×100 μm, film thickness<100 nm). The drop-on-demand mode may be more useful for the depositionof high-resolution droplets (e.g., droplet diameter <2 μm, dropletheight <100 nm). In a continuous jet-printing mode, high-resolutiondroplets are generated by increasing the applied voltage, whichsimultaneously results in higher frequency jetting. High stage speed isthen used to space out the printed material on the substrate to formsessile droplets rather than large conglomerations of printed droplets.As the stage speed increases, additional dynamics and noise may beintroduced into the process, thus reducing the overall quality of theprinted patterns. Thus, drop-on-demand printing offers more stability ata particular spatial resolution by controlling the release of a smallvolume of material at a desired coordinate and at a desired time.

Multi-material, layered structures with well-defined areas, smoothinterfaces between layers, and controllable thicknesses may befabricated using the disclosed methods. As described below, thedisclosed methods provide a systematic technique to achieve thesestructures by e-jet printing.

The e-jet printing employs a complex ejection mechanism that is affectedby the fluidic properties of the build material (e.g., surface tension,electrical conductivity, viscosity, density, etc.) and processparameters (e.g., nozzle size, nozzle electrical resistance, appliedelectric field, the surface energy of the substrate, etc.). Thechallenges that these and other aspects of the e-jet printing presentfor the printing process are addressed below. Material interactions atthe micro and nanoscale are also addressed. Various combinations ofprocess parameters and material properties are described in connectionwith the fabrication of multi-material, multi-layer structures withcontrol of thickness at the nanoscale, and control of in-planepatterning at the microscale.

The deposition of individual droplets is described, along with how thedroplets merge to form continuous layers. Those results are then appliedto the fabrication of multiple stacked layers of different materials,enabling structures like vertical Bragg reflectors, examples of whichare depicted in Part B of FIG. 13 . The reflectance of a Bragg reflectorcan be tuned by the number of layers and corresponding thicknesses, asshown in Part C of FIG. 13 , given a set refractive index material pair.Several process parameters that determine the quality of film formationare then addressed, which in turn influences device functionality. Thematerial properties and their scale-dependence that contribute tohigh-quality film formation are also addressed.

A number of materials are compatible with the disclosed methods, in thesense that the materials demonstrate stable jetting behavior duringdeposition as well as merging characteristics after the material hasreached the substrate. Stable jetting behavior may describe materialsthat form a stable, single Taylor cone jet at the meniscus withoutclogging or evaporating. Good merging characteristics may describe buildmaterials that spread to a uniform thin-film on existing topography.These characteristics may be useful in connection with the fabricationof a photonic crystal or other photonic structure or device using e-jetprinting, e.g., with alternating layers of low and high refractiveindices of commercially available photopolymers. Examples of materialcombinations with favorable optical properties for the fabrication of amulti-material, multi-layer photonic crystal are also presented below.The photonic responses of the example structures are also quantifiedbelow.

Thin-film fabrication using the drop-on-demand e-jet printing of thedisclosed methods includes or involves droplet ejection, dropletspreading, and droplet coalescence. Materials with stable jettingbehavior will form a single stable Taylor cone jet at the meniscus.After the material is ejected from the nozzle tip, a sessile droplet isformed on the substrate (or other layer) with a spherical cap shape thatis defined based on the droplet diameter and contact angle. The dropletgeometry depends on the electric field, the kinetic energy imparted onthe droplet at ejection, the surface tension of the droplet, surfaceenergy of the substrate, rheological properties of the ink, and theviscous energy lost during spreading. UV-curable photopolymer inks maybe deposited in this manner onto conductive, smooth silicon wafers. Theinks may be selected for their combination of fluid properties as wellas having the ability to be cured in situ, without requiring hightemperature operations. As an example, NOA170 may be used in some casesto form thin-films on a polished silicon substrate.

The controllable process parameters that can affect the applied electricfield, and subsequent droplet volume, include: high voltage value, lowvoltage value, pulse width (t_(p)), nozzle size, and standoff height(distance between the nozzle tip and the substrate). The droplet volumeof a specific material may be adjusted using the pulse width t_(p),while keeping all other process parameters constant. The pulse widtht_(p) may be used in this manner because the pulse width has a directmapping to droplet volume, and reduces the introduction of additionaljetting dynamics and disturbances, such as nozzle arcing, which are morelikely to occur with changes in other process parameters like highvoltage or standoff height. Each droplet may be a result of a singledroplet released within the designated pulse width t_(p). Successivedroplets may be placed at a certain distance (center to center) fromeach other, defined as pitch, to form a film. The average thickness of afilm, g, and the root mean square (RMS) surface roughness, s_(q), may bedefined as:

$\overset{¯}{g} = {\frac{1}{N_{1} \times N_{2}}{\sum\limits_{i = 1}^{N_{1}}{\sum\limits_{j = 1}^{N_{2}}g_{i,j}}}}$$s_{q} = {\frac{1}{N_{1} \times N_{2}}\sqrt{\sum\limits_{i = 1}^{N_{1}}{\sum\limits_{j = 1}^{N_{2}}( {\overset{\_}{g} - g_{i,j}} )^{2}}}}$

where g_(i,j) is the topography of the pattern at the discretizedcoordinate of (i,j), and N₁ and N₂ are the total number of discretizedcoordinates in the X and Y direction. Note that both g and s_(q) aremeasured using an integrated AFM.

Roughness is a representation of the merging quality of a film such thata low s_(q) value indicates a smoother film. The thickness and roughnessof the printed films may be regulated by adjusting the droplet volumeand pitch. As such, pitch and t_(p) are independent variables while filmthickness and roughness are dependent variables. At small t_(p) or largepitch values (pitch much greater than D), the droplets become smallerthan the pitch, which yields voids in the pattern and increases thefilm's roughness. To quantify film quality, a thin-film pattern with athickness smaller than 200 nm is considered to be fully merged if it hasan s_(q) value less than 20 nm, partially merged for s_(q) valuesbetween 20 nm and 50 nm, and unmerged for s_(q) values greater than 50nm.

Several examples of these interactions were tested at the microscale.Loctite3526 fully merges on NOA170 with thickness and RMS roughness of90 nm and 6 nm, respectively. NOA170 partially merges on NOA144 withthickness and RMS roughness 175 nm and 40 nm, respectively. NOA170 doesnot merge on NOA1348 with thickness and RMS roughness 250 nm and 200 nm,respectively.

With reference again to FIG. 11 , Part A shows the effect of pulse widthtp on the average thickness and corresponding RMS surface roughness(represented as error bars) of NOA170 films (e.g., 60 μm×60 μm). In thisexample, the controlled process parameters include: V_(h)=500 V,V_(l)=250 V, nozzle size=1 μm, standoff height=20 μm, and pitch =2.4 μm.The pulse width, t_(p), may be varied. In this example, the pulse widthfell in a range from about 0.5 ms to about 12 ms. The range may be usedto investigate its effect on film quality. Note that the diameter of adroplet is varied by changing t_(p). It is observed that decreasingt_(p) from 12 ms to 2 ms decreases the pattern thickness and roughness.A pulse width of 1 ms (t_(p)=1 ms) resulted in the lowest roughness(s_(q)=7.23 nm) with a film thickness of 95 nm. Other pulse widths maybe used.

Part B of FIG. 11 shows the influence of pitch on average thickness androughness of NOA170 films. In this example, a raster type motion is usedto print a continuous line that merges to form films, as presented inthe inset of Part B. The controlled process parameters are the same asthose used to evaluate the effect of pulse width; however, pulse widtht_(p) is set to 1 ms, which yields an average droplet diameter ofD=2.83±0.12 μm. The pitch was varied in a range of 1-2.9 μm. As thepitch increases, both the roughness and thickness decrease. In thiscase, pitch of 2.5 μm resulted in the lowest roughness (s_(q)=8.41 nm)with an average thickness of 89.63 nm. Other pitches may be used.

The above-described examples show that the e-jet printing process of thedisclosed methods may be customized via at least two parameter selectionsteps. The process may include (1) ejecting controlled droplets bypulsing from low to high voltage over a designed time period (a shorterpulse width t_(p) leads to smaller droplets), and (2) adjusting thepitch between deposited droplets to achieve thin, uniform patterns. Insome cases, the pitch values may equal or otherwise correspond with(e.g., be similar to) the droplet diameter D.

The process parameters may be selected to achieve the deposition ofsmooth, nanoscale films onto a uniform surface (e.g. polished silicon).However, in some cases, tuning the process parameters does not guaranteea fully merged film. It is possible for a build material to merge into auniform film on one material, but not on a different material. Thisraises the question of how to determine appropriate materialinteractions in multi-material structures that are fabricated in alayer-by-layer fashion. For example, to create a multi-materialstructure such as a photonic crystal, a refractive index contrast(Δn=n_(H)−n_(L)) is established between neighboring layers, whichintroduces variations in surface energy for each additional layer beingdeposited. To quantify the impact of these variations, examples of theshape of sessile droplets of a build material on a previous surface weretested. A number of photopolymers were tested: NOA170, Loctite3526,NOA144, NOA142, NOA13825, NOA138, NOA13775, NOA1369, and NOA1348, withrefractive indices ranging from n=1.35 to 1.71, which provides an indexcontrast maximum of Δn=0.35. In one example, NOA170 was the n_(H)material due to its high refractive index of 1.70. A suitable lowrefractive index material (n_(L)) may then be found to fabricate amulti-layer structure such as those described herein. The substrate(e.g., a silicon wafer) may be referred to as the primary substrate,while previously deposited, fully merged and cured photopolymer surfacesmay be referred to as a secondary support layer.

The substrate/support layer-ink interaction may be defined or otherwiseestablished by the contact angle, which is a function of fluidicproperties of the build material (Liquid surface tension (LST),viscosity, evaporation rate, density, etc.), and surface energy of theprevious (substrate or supporting) layer. The solid surface energy (SSE)of a substrate may affect droplet shape and subsequent featureresolution in high-resolution e-jet printing, where the microscalecontact angles of droplets may be used to predict the merging quality oflines on varying SSE surfaces. Microscale contact angles (e.g., dropletsof 2-10 μm) may also be used to predict or establish the roughness of adeposited layer of a build material on the surface of a previouslyprinted material. Microscale contact angles have been shown to differfrom macroscale contact angles due to factors such as surface chemicalheterogeneity and surface structural heterogeneity

FIG. 14 presents data regarding these microscale interactions. In PartA, NOA170 is printed on the primary substrate as the n_(H) material, andthe n_(L) materials are printed on top of a secondary support layer ofNOA170 (unfilled markers in FIG. 14 , Part A). NOA170 is printed onsecondary support layers of n_(L) photopolymers (filled markers in FIG.4A). The low refractive index materials in these examples include:Loctite3526, NOA144, NOA142, NOA13825, NOA138, NOA13775, NOA1369, andNOA1348. Three regimes defined by the roughness of the formed film(e.g., 60 μm×60 μm) of a build material on a substrate are delineatedfor ease of characterization. A monotonically increasing linearrelationship on a log-log plot is found between the contact angle of asingle droplet (2-10 μm) of a build material on a substrate (primarysubstrate/secondary layer of a different material) and the roughness ofa printed film of the same build material on the substrate. It isobserved that build materials with a low contact angle have a higherlikelihood of adhering to previous surfaces and forming a uniformthin-film on them. Focusing on the microscale measurements in Part A ofFIG. 14 , a printed layer of NOA170 serves as the secondary supportlayer for the deposition of low index materials. All low index inksexhibit fully merged thin-films (less than 200 nm) with low contactangles (less than 15°) and low RMS surface roughness (less than 10 nm).Interestingly, the deposition of NOA170 on top of low refractive indexmaterials does not perform as smoothly. Depending on the material in theprevious (supporting) layer, the contact angles range from 10° to 50°with resultant pattern roughness values ranging from less than 20 nm togreater than 200 nm. One example of a low index material that NOA170fully merges onto is filtered Loctite3526, while NOA170 printed ontoseveral other low index ink secondary support layers shows partialmerging.

The results shown in Part A of FIG. 14 describe material spreading on auniform surface with low roughness (e.g., less than 10 nm). At themicroscale, in addition to the topology, chemical heterogeneity of theprevious layer may play a role in surface roughness. The thickness ofthe current layer may be affected by the roughness of the previouslayer.

The effects of process parameters may be combined with the foregoingresults to achieve suitable material spreading of printed layers ofpolymers. For instance, the spreading may be a function of processparameters and the contact angle of printed single droplets. Additionalor alternative liquid inks may be used, including, for instance,nanoparticle-based liquid inks. Alternative or additional materialproperties, such as density, viscosity, and conductivity, may also beused to control material spreading for a broad range of materials at themicroscale.

Macroscale surface energetics were tested to further explore materialspreading. While the disclosed methods may focus on controlling materialbehaviors at the microscale, measuring SSE at these length scales isquite difficult. Surface energetics at the macroscale may thus beuseful. The results are summarized in Part B of FIG. 14 . The siliconwafer showed the highest average SSE (

_(s)=66.3 mN m⁻¹), followed by NOA170 (

_(s)=48.3 mN m⁻¹), while the lower index materials (n_(L)=1.35-1.51)exhibited significantly lower SSE values (

_(s)=11.5-19.5 mN m⁻¹). A material with a high SSE value may be a morefavorable substrate for realizing uniform film formation of the nextlayer. This supports the observations that NOA170 is a favorablesubstrate for the examples of low index materials addressed herein.

The LST values of the example inks were evaluated using the pendantdroplet method. An ink may be considered to be a highly cohesive ink ifthe ink has a high LST value and exhibits poor wetting behavior due to apreference for attaching to itself rather than adhering to a substrate.On the other hand, poorly cohesive inks (low LST values) are not able toremain bonded to themselves to form a uniform pattern on existingtopographies. Based on the values provided in Part B of FIG. 11 , NOA170has a relatively high LST value (37.3 mN m⁻¹). Note that NOA170 spreadsreadily on a silicon substrate (

_(s (silicon))>

_((NOA170))), but exhibits mixed merging behavior on lower SSE valuedsurfaces. A highly cohesive ink (e.g., NOA170) is unlikely to adhere toa low surface energy substrate, (

_(l (NOA170))>

_(s (nL materials))). In some cases, these interactions may be addressedas follows. First, the SSE value of a merged layer of an n_(L) materialmay be increased using in-situ modifications such as atmospheric plasmatreatments. Second, efforts may be spent in determining methods fordecreasing the LST of a high index material (e.g., NOA170) to promoteimproved merging quality.

The material interactions at the microscale are a result of a trade-offbetween contact angle, SSE, and LST values. For example, printeddroplets of NOA170 on NOA1348 and NOA138 secondary support layersexhibit similar contact angles at the microscale (Part A of FIG. 14 ).However, a lower SSE value for NOA1348 may address why NOA170 fails toresult in a merged film on this surface. The filtered Loctite3526 andNOA13775 have similar SSE values; however, the higher contact angle ofprinted droplets of NOA170 on the printed NOA13775 surface results in arougher surface deposition for films of NOA170. In addition, partialmerging was observed with the deposition of NOA170 on NOA138, NOA142,and NOA144; which also showed moderate contact angles of printed NOA170droplets (15°<θ<25°) as compared to NOA1369 with similar SSE values.

FIGS. 15 and 16 present the results of the fabrication of a number ofexamples of multi-layer thin-film structures. The structures maycorrespond with, or form a portion of, one or more of the photonicdevices described herein.

Part A of FIG. 15 shows multi-material microstructures that werefabricated by e-jet printing of two high viscosity adhesives at roomtemperature in accordance with one example. In this case, thefabrication method was implemented and configured to deposithigh-resolution NOA170 and NOA13825 patterns with layers of 100 nm and200 nm average thickness, respectively. The darker color patterns were20×20 μm films of NOA170 (4400-5000 cP) with an average thickness andRMS roughness of 101 nm and 5 nm, respectively. The lighter colors weredeposited 17×17 μm films of NOA13825 (5600 cP) with an average thicknessand RMS roughness of 205 nm and 12 nm, respectively. The distancebetween the patterns was set at 5 μm. The pattern profile across thelast row of the printed structure (shown in Part B of FIG. 15 )highlights the flexibility and repeatability of the e-jet printingprocess.

The multi-material, multi-layer fabrication of two high viscositymaterials using e-jet printing is presented in Part C of FIG. 15 .NOA170 was printed in layers 1 and 3, while filtered Loctite3526 wasprinted in layer 2. To achieve the desired effect, each layer isdesigned to be approximately 160 nm thick. The e-jet process parametersfor NOA170 were V_(h)=600 V, V_(l)=200 V, t_(p)=1 ms, f=20 Hz, andpitch=1.8 μm. The e-jet process parameters for Loctite3526 wereV_(h)=500 V, V_(l)1=250 V, t_(p)=5 ms, f=20 Hz, and pitch=2 μm. At eachlayer, the liquid patterns were UV-cured and their topography wasmeasured using the integrated AFM.

Part C of FIG. 15 shows the corresponding average total height map overfive pixels from the middle of the pattern at each layer. The overallvariation (roughness/total height) in the total stack height was lessthan 6% across a single layer and 4% across the entire stack. Theaverage total height is 159±9 nm for layer 1, 325±13 nm for layer 2, and489±17 nm for layer 3, respectively. The maximum RMS roughness in alllayers is less than 17 nm, which is a demonstration of the flatness inthe overall height. The integration of control to the e-jet process maybe used to mitigate height variations.

FIG. 16 depicts an example of a high-resolution e-jet printed Bayerfilter array using a high refractive index polymer (NOA170, n=1.7) and amedium refractive index material (Loctite3526, n=1.51). Patterns 1, 2,and 3 are associated with the red, blue, and green color spectrum,respectively, and are equally spaced with a 15 μm offset with roughnesssmaller than 13 nm. As shown via these bi-layer samples, the e-jetprocess may independently control layer thickness, regardless of theprevious printed layer thickness, with the end result being control overthe reflected light intensity at specific areas of the spectrum. Thus,the following layer goals were set with the first and second layers at90 nm and 130 nm for pattern 1, 130 nm and 180 nm for pattern 2, and 90nm and 180 nm for pattern 3, respectively. These combinations achievedred, green, and blue reflected peak intensity with differingcombinations of thickness of both the NOA170 as base layer andLoctite3526 as the second layer. Through drop-on-demand e-jet printing,the thickness of each layer is controlled precisely and by design. Asdemonstrated, it is possible to create a variable color spectrum usinge-jet printing, as the color spectrum is correlated with the layerthickness and the corresponding refractive indices of each layer.Therefore, for a fixed material combination, the optical properties maybe varied by adjusting the layer thickness.

The above-described examples show that the thickness variation is within6% across a single layer. Different factors may affect these variations,including, for instance, (1) the measurement apparatus has ±10 nmthermal noise that directly affects the roughness measured, (2)commercial inks may contain large particles that increase thicknessvariations, and (3) the e-jet is an iteration varying process anddifferent factors such as nozzle clogging, environment temperature andhumidity, and more can affect the deposition process and eventuallyaffect the roughness.

FIG. 17 depicts a method 1000 of fabricating a photonic device inaccordance with one example. The method 1000 may include fewer,additional, or alternative acts. For example, the method 1000 mayinclude any number of printing and curing acts. The method 1000 may alsoinclude one or more acts directed to forming a photodetector or otherdevice on the substrate. An ink filtering act may or may not beimplemented.

In the example of FIG. 1 , the method 1000 may begin with an act 1001 inwhich one or more detectors are formed on a substrate. In some cases,the substrate is composed of, or otherwise includes, silicon. Thecomposition and/or other characteristics may vary. For instance, thesubstrate may be composed of, or otherwise include, glass, indium tinoxide (ITO) coated glass, and/or other materials.

The act 1001 may include forming a photodetector on the substrate suchthat a first layer of the stack is supported by the photodetector. Theact 1001 may alternatively or additionally be directed to otherwisepreparing the substrate and/or other structures on which thephotopolymer stacks are disposed.

In some cases, one or more photopolymer materials are filtered in an act1002. For instance, a first photopolymer may be filtered in preparationfor printing a first layer. The act 1002 may include alternative oradditional photopolymer preparation steps.

In an act 1004, a first layer composed of, or otherwise including, afirst photopolymer is printed such that the first layer is supportedindirectly or directly by the substrate, as described herein. Theprinting uses an electrohydrodynamic jet (e-jet) printing apparatus, asdescribed herein. In some cases, the e-jet printing apparatus isoperated in a drop-on-demand mode in an act 1006. The act 1004 mayinclude an act 1006 in which a pulse width and/or frequency of the e-jetprinting apparatus is controlled. The act 1004 may include controlling avoltage of the e-jet printing apparatus in an act 1010 and/or a pitch ofdroplets deposited by the e-jet printing apparatus in an act 1012.

In an act 1014, the first layer is cured. In some cases, the first layeris cured in an act 1016 while on the e-jet printing apparatus. The act1014 may include an act 1018 in which the first layer is cured using UVlight. Alternatively or additionally, the first layer may be cured in anitrogen flow in an act 1020.

In some cases, the method 1000 includes an act 1021 in which thereflectance of the first layer is measured before printing a secondlayer. In some cases, the measurement is implemented using amicrospectrometer integrated with the e-jet printing apparatus.

In an act 1022, a second layer is printed using the e-jet printingapparatus such that the second layer is supported by the first layer.The second layer is composed of, or otherwise includes, a secondphotopolymer, as described herein. The act 1022 may include any one ormore of the printing steps or procedures described herein, e.g., such asin connection with printing the first layer.

The second layer is cured in an act 1024. Curing the second layer mayinclude any one or more of the curing steps or procedures describedherein, e.g., such as in connection with curing the first layer. In somecases, curing of the first layer and/or curing of the second layer areimplemented while the substrate remains on the e-jet printing apparatus.

The method 1000 may then include an act 1026 in which theabove-described printing and curing acts are repeated. For instance, theacts may be repeated to print one or more pairs of layers including thefirst and second photopolymers such that the one or more pairs of layersare supported by the second layer.

The order of the acts may vary from the example shown. For example, thefiltering of the photopolymers may be implemented sequentially, e.g.,after the previous layer has been deposited and/or cured. In anotherexample, further printing of a first layer (act 1004) may be implementedafter the topography of the device is assessed (and/or reflectance ismeasured) using an inline or integrated microspectrometer (act 1021)before proceeding with the printing of a second layer. Any number ofreflectance measurements using the microspectrometer may be implementedduring the procedure due to the integrated nature of themicrospectrometer.

FIG. 18 depicts of an e-jet printing system 1800 in accordance with oneexample. In this case, the e-jet printing system 1800 includes an inlineor integrated microspectrometer 1802. The e-jet printing system 1800 maybe used to implement the method of FIG. 17 . The e-jet printing system1800 includes an e-jet printing apparatus 1804, a curing apparatus 1806,and the microspectrometer 1802, each being disposed at a respectivestation. In operation, the substrate on which the photonic device isbeing fabricated may be shuttled (depicted via arrows) between thestations to implement the method.

In the example of FIG. 18 , the microspectrometer 1802 includes a fiberoptic cable 1, a fiber zoom housing 2 for focusing spot size, an XYstage 3 for fiber sample spot adjustment (e.g., for microscaleprecision), a beamsplitter 4 for image acquisition (e.g., a 90/10 (R:T)visible beamsplitter), a CMOS camera 5, which may be monochromatic orcolor, variable R:T beamsplitters 6 (e.g., UV, visible, NIR, MIR),variable light source 7 (e.g., halogen, fluorescent, visible, IR, etc.),which may include a laser source or an interferometer for Raman FTIRmeasurements, an objective zoom housing 8 for focusing objective, avariable microscope objective 9 with refractive and reflective options,and one or more variable spectrometers with various wavelength detectionranges (e.g., 300-850 nm, 900-1600 nm, etc.).

In operation, inline or integrated microspectroscopy is provided bypassing light from the light source 7 to the beam splitter 6. The splitbeam is then focused by the objective zoom onto the sample surface andthen used to collect the reflected light. The reflected light passesback through the beam splitter 6 and then onto the beamsplitter 4. Aportion (e.g., 10%) of the light is sent to the camera 5, while theremainder is set to the spectrometer for processing.

The integration of the microspectrometer 1802 permits in situmeasurement of optical properties (e.g., reflectance) of the patternsbeing printed, and thereby enables rapid iteration of the structure andprinting parameters to achieve a desired outcome.

Various photopolymer materials may be used. In one example, thematerials include one or more optical adhesives, such as adhesivesavailable from Norland Optical Products (NOA) as well as a commercialLoctite formulation.

The e-jet printing apparatus 1804 may be configured for high-resolutionpatterning. For instance, nozzles smaller than 1 μm in inner diametermay be used with a 20 μm standoff height.

The method may include one or more filtering acts. Some inks containlarge particles (e.g., comprising resin, a long chain oligomer, orforeign moieties). Such particles may be removed via filtering in orderto reduce the chance of nozzle clogging. For example, Loctite3526 may befiltered using high pressure and a filter with 0.22 μm diameter pores.The removal of large particles may also reduce the surface roughness ofthe printed patterns.

Filtering may, in some cases, change other ink properties. For example,filtered NOA170 has a smaller refractive index value than unfilteredNOA170, while also exhibiting an unstable spray jet instead of a singlestable jet mode at the same standoff height and voltage.

The printing acts may be implemented in an ambient atmosphere. Thecuring acts may be implemented in a nitrogen atmosphere due to oxygeninhibition of the photopolymerization of one or more of the inks(excluding Loctite 3526). This may be achieved by creating an enclosurearound the LED curing bulb and flowing nitrogen at a high rate over thesurface.

Described above are e-jet printing-based methods of fabrication thatutilize the merging quality of UV-curable polymers to form thin-film,multi-material, layered microstructures. Materials and process criteriamay be selected to achieve material ejection as well as material mergingin the e-jet printing. The e-jet printing-based methods may serve as asubstitute for other manufacturing techniques, such as lithography, tofabricate high-resolution photonic devices that are made of multiplethin layers of different materials. Microscale, high-quality films werefabricated with the following material combinations (1) low contactangle, (2) high surface tension of the build material, and (3) highsurface energy of the previous layer. The controllability andrepeatability of e-jet printing were demonstrated by fabricating a Bayerfilter of multiple colors using a drop-on-demand printing mode of thee-jet printing.

Novel structures (e.g., photonic and electronic structures) with usefulthermal, electrical, and optical properties may be fabricated by thee-jet printing-based methods. The structures may include multi-material,multi-layer films with microscale spatial resolution and nanoscalethickness control. The structures may be configured to form onedimensional photonic crystals (1DPCs) with a response near, e.g., thevisible regime. Photopolymers with varying refractive indices (n=1.35 to1.70) may be used based on their relative high index contrast and fastcuring times. The structures may be combined to provide pixelated 1DPCswith individual layer thicknesses between 80-200 nm, square pixelssmaller than 40 μm across, with surface roughness less than 20 nm.

The present disclosure has been described with reference to specificexamples that are intended to be illustrative only and not to belimiting of the disclosure. Changes, additions and/or deletions may bemade to the examples without departing from the spirit and scope of thedisclosure.

The foregoing description is given for clearness of understanding only,and no unnecessary limitations should be understood therefrom.

What is claimed is:
 1. A device comprising: a substrate; and a patternedstack supported by the substrate, wherein the patterned stack comprises:a first layer supported by the substrate, the first layer comprising afirst photopolymer; and a second layer supported by the first layer, thesecond layer comprising a second photopolymer; wherein the first andsecond photopolymers have different refractive indices.
 2. The device ofclaim 1, wherein the patterned stack further comprises furtheralternating patterned layers of the first and second photopolymers, thealternating patterned layers being supported by the second patternedlayer.
 3. The device of claim 1, wherein each of the first and secondpatterned layers has a thickness less than about 200 nm.
 4. The deviceof claim 1, wherein the patterned stack is configured as aone-dimensional photonic crystal.
 5. The device of claim 1, wherein thepatterned stack is configured as a Bragg reflector.
 6. The device ofclaim 1, wherein the patterned stack is one of a pixelated array ofpatterned stacks supported by the substrate.
 7. The device of claim 1,further comprising a photodetector supported by the substrate.
 8. Thedevice of claim 7, wherein the photodetector is disposed between thepatterned stack and the substrate.
 9. The device of claim 1, wherein thesubstrate comprises silicon.
 10. The device of claim 1, wherein thefirst photopolymer has a higher refractive index than the secondphotopolymer.
 11. The device of claim 1, wherein the patterned stackfurther comprises a third layer supported by the second layer, the thirdlayer comprising a third photopolymer.